This invention relates to a process for treating a copper surface suitable for making copper on an internal layer circuit board adhere to a prepreg in a step for laminating internal layer circuit boards and prepregs alternately for producing a multi-layer printed wiring board.
Multi-layer printed circuit boards have been produced by removing unnecessary portions of copper foils of upper-clad laminates by, for example, etching to form internal layer circuits, roughening the copper surfaces of internal layer circuits with a chemical fluid, subjecting the roughened surfaces to oxidation treatment, piling prepregs and copper foils which become outer layer circuits on a plurality of internal layer circuit boards and laminating them into one body, drilling holes at portions necessary for connecting the outer layer circuits and the internal layer circuits, removing resins and pieces of glass sticked to surfaces of the holes with a chemical treating fluid, forming a metal layer such as a copper layer by electroless plating or the like, and forming outer layer circuits by etching or the like. In such a process, the oxidation treated layer of copper surface exposed on the surfaces of holes is easily corroded by the acidic chemical treating fluid in a later step to form portions wherein no adhesion is maintained between the internal layer copper foils and prepregs around the holes (so-called pink rings), which results in bringing about peeling by a thermal shock such as soldering.
In order to form a surface hardly dissolved with the chemical treating fluid which is more acidic than copper oxide, Japanese Patent Unexamined Publication No. 56-153797 discloses a process wherein the copper oxide formed is reduced by dipping in an alkali solution such as a solution of sodium borohydride, formaldehyde or the like. But when copper oxide is reduced by using a sodium borohydride solution, the surface appearance is blurred after the treatment and adhesion of prepregs often becomes insufficient. On the other hand, when an aqueous solution of formaldehyde is used, there takes place a problem that the treating rate is small.
On the other hand, in order to improve adhesion between a metal and a resin in the production of multi-layer printed wiring boards, there are proposed the use of an amineboran as a reducing agent for copper oxide (Japanese Patent Unexamined Publication No. 61-176192), the treatment of copper oxide with an activating solution including a catalytic metal element before the reduction to copper (Japanese Patent Unexamined Publication No. 61-279531), the use of a reducing agent solution dispersing metal particles having a catalytic ability therein (Japanese Patent Unexamined Publication No. 61-266228), and the exposure of metallic copper on a part of a copper surface covered with a copper oxide layer when the copper oxide layer is reduced with a reducing agent solution (Japanese Patent Unexamined Publication No. 61-250036). But the adhesive strength between the metal (copper) and the resin is still insufficient for practical use according to these proposals.